1. Field of the Invention
The present invention relates to an electric rotating machine and, more particularly, to an electric rotating machine that can be used for a starter for cranking engines.
2. Related Art
A surface-commutator type electric rotating machine is proposed in U.S. Pat. No. 5,508,577 (JP-A7-231617). In this electric rotating machine, coil ends of an armature coil extend towards a rotary shaft of an armature in the radial direction along the axial side surfaces of an armature core so that the coil ends disposed at one axial side of the armature core are used as commutator surfaces of a commutator on which brushes slide. As the radially extending coil ends are used as the commutator, the axial length of the armature can be shortened and hence the size of the electric rotating machine can be reduced.
In manufacturing this type of electric rotating machines of different output power specifications, it is more advantageous to change the axial length of the armature than to change the radial length of the armature from the standpoint of using same component parts for the electric rotating machines and same manufacturing apparatus for manufacturing the electric rotating machine.
Provided that only the axial length of the armature is changed for a different output, the heat capacity of the armature will increase with increase in the axial length of the armature. Further, provided that the electric current supplied to the brushes is increased for higher output power, density of current in the brush-commutator sliding area increases correspondingly. As a result, the sliding of the brushes on the commutator surface increases heat generation, increasing the temperature of the entire part of the armature. This temperature rise tends to increase the electric resistance of the armature coil, causing lower output power than desired as well as shorter longevity of the brushes and the coil ends used as the commutator.
Therefore, it becomes necessary to enlarge the diameter of the armature to assure an increased brush-commutator sliding area. This will cause increase in the size of the armature in the radial direction and in the inertia mass of the armature. In the case where this electric rotating machine is used for a starter, for instance, the increased inertia mass of the armature will produce an increased collision shock when the starter engages with the ring gear of an engine.
Further, as the armature that uses the coil ends as the commutator has a shorter length in the axial direction of the armature core, it becomes difficult to dissipate the heat to the surrounding space. Thus, the heating of the armature coil must be reduced to maintain the electrical insulation of the armature coil. Particularly, when the conductors of the armature coil disposed in slots of the armature core expand in the axial direction because of heat expansion, the coil ends tend to move in the axial direction. Therefore, it becomes necessary to cool the armature coil for restricting the heat expansion.